1. Field of the Invention
This invention relates to a headphone apparatus, and more particularly to a headphone apparatus having a function of canceling noise from the outside.
2. Description of the Related Art
A headphone of the type wherein a noise component which comes into a cavity between a headphone unit of a headphone and an ear from the outside is detected by means of a microphone and an output signal of the microphone is fed back to a loudspeaker of the headphone unit to reduce noise from the outside is already known. The operation principle of such noise cancellation operation of the headphone is known as feedback theory based on the Bode's theorem. In particular, the attenuation amount of noise and the bandwidth of frequencies in which noise can be canceled are characterized by an open loop characteristic.
Generally, in order to stabilize a closed loop characteristic to prevent oscillations or the like, it is required that the attenuation characteristic outside a frequency band in which noise can be canceled (hereinafter referred to merely as the noise canceling band) be -6 dB/oct to -12 db/oct.
In the meantime, also a technique of making the attenuation characteristic outside a canceling band substantially flat has been proposed as disclosed, for example, in U.S. Pat. No. 4,455,675.
However, an increase of the active band, that is, the noise canceling band of a wide band reproduction headphone which can reproduce sound in a frequency band of substantially 20 Hz to 10 KHz cannot be realized if the gain characteristic of the open loop characteristic of a feedback loop outside the noise canceling band is set to a dropping characteristic or to a substantially flat characteristic as described above. This is because, when the noise canceling band is widened, oscillations occur in a high frequency portion of the noise canceling band. One kind of wide band reproduction headphones is a headphone of the non-reflective type wherein an acoustic pipe having an ear mounting portion at an end portion thereof and a non-reflective end for sound at the other end thereof and having an inner diameter substantially equal to an external auditory canal is provided and a headphone unit is mounted on the acoustic pipe such that a sound emitting face thereof is opposed to the inside of the pipe. A non-reflective headphone of the type just described is disclosed in U.S. Pat. No. 5,022,486.